Convergent and Parallel Evolutionary Traits in Early Cretaceous Rudist Bivalves (Hippuritidina)

Abstract

Early Cretaceous Hippuritida clades, requieniide (family Requieniidae) and hippuritide (families Radiolitidae, Polyconitidae,
Caprinidae, “Caprinulidae” and Caprinuloideidae), show distinctive myophoral arrangements and shell structures.
Nevertheless they share some characters, such as the transverse shell thickening of the myophores of the attached valve
which are convergent traits in Lovetchenia (Requieniidae) and Homopleura (Monopleuridae). The bent posterior myophore
of the right valve of Pseudotoucasia (Requieniidae) closely resemble the posterior myophore of the left valve of Horiopleura
and Polyconites (Polyconitidae). The shell cellular structure is one of the key attributes of the family Radiolitidae (e.g.
Eoradiolites) but this structure is also present in some advanced Requieniidae (“Toucasia-Apricardia “group). Canaliculate
shell structures are convergent evolutionary traits which are common in the Caprinidae and Caprinuloideidae and also exist in
the Polyconitidae and “Caprinulidae”. In most of the foregoing canaliculated groups, two trends are well expressed, reflecting
parallel evolution: expansion of canals into the entire shell and increasing complexity of canal architecture. Convergent taxa
took some advantages by using former innovations. An Albian peak of convergence coincided with the emergence of new
clades, which suggests a reset following the mid-Aptian extinction event.